1. Field of the Invention
The present invention relates to a method of checking defects in photo masks used, for example, in various types of pattern formation techniques, etc. in the production of semiconductor devices.
2. Description of the Related Art
A photo mask, which is used in the pattern transfer process, i.e., the so-called lithography process, in the production of semiconductor devices, is used for the purpose of transferring a pattern formed thereon to a resist material formed on a wafer. It is usually impossible to completely eliminate defects in the pattern formed on the photo mask, so that pattern defect checking is performed mainly by a method which is described below. In the following description, the term "pattern" means a pattern formed on a photo mask unless it is defined otherwise.
First, the photo mask is irradiated with rays of light, and those rays of light transmitted through the pattern are detected by a photo detector such as a CCD sensor or a photo multiplier, the pattern being reproduced as a light intensity distribution. If there is any defect in the pattern, the light intensity distribution reproduced as the pattern indicates various changes depending upon the size or type of the defect. These changes in the light intensity distribution are recognized as a defect by the following two defect checking methods:
The first defect checking method (which is called a "die-to-die method") is applied to a case in which a plurality of patterns of the same configuration exist on a photo mask. In this first defect checking method, the light intensity distribution which has changed due to the pattern defects is compared with a light intensity distribution formed by a pattern of the same configuration having no defect and arranged in a different section of the photo mask. Any defect in the pattern is recognized as such by the difference between these light intensity distributions.
In the second defect checking method (which is called a "die-to-data-base method"), the light intensity distribution which has varied due to a pattern defect is, as shown in the pattern defect checking flowchart of FIG. 1, compared with pattern data, and any defect in the pattern is recognized as such by the difference between the light intensity distribution and the pattern data. Here, the pattern data is first used to form a pattern on a photo mask by an electron beam, a laser beam or the like. In the defect checking, this pattern data is used for comparison with the light intensity distribution.
The conventional checking methods have the following problems:
As a result of the recent increase in the chip size of semiconductor devices, it is becoming impossible to arrange a plurality of patterns of the same configuration on a photo mask, which means it is only possible to form a single pattern on a photo mask. Thus, it is becoming practically impossible to adopt the first defect checking method. As a result, there is nothing for it but to adopt the second defect checking method. However, this second defect checking method has the following problems:
First, nowadays, the pattern size is approaching the wavelength of the exposure light of an exposure device such as a stepper. As the pattern size is thus reduced, it becomes impossible for the configuration of the pattern formed on the photo mask to be faithfully transferred to the resist formed on the wafer due to a physical phenomenon called light proximity effect. In view of this, a correction process which is generally called a light proximity effect correction is effected on the pattern in order to achieve an improvement in the faithfulness with which the pattern formed on the photo mask is transferred to the resist formed on the wafer.
This light proximity effect correction can be effected in various ways. Usually, the following methods are adopted: a method in which a minute pattern is added to the pattern; a method in which a minute pattern is deleted from the pattern; a method in which a minute pattern is added to the vicinity of the pattern; and a method in which the pattern size is locally increased or decreased. This light proximity effect correction is performed for the purpose of transferring a desired pattern configuration as faithfully as possible to the resist formed on the wafer by correcting the pattern. Generally speaking, the correction is conducted in many cases with a minute pattern which is of a size smaller than the wavelength of the exposure light. In a light intensity distribution obtained from a pattern corrected through light proximity effect correction, the minute correction pattern and the local increase or decrease in pattern size are not reproduced faithfully due to light diffraction effect. Thus, a difference corresponding to the correction pattern is inevitably generated between the pattern data, which is faithfully endowed with a minute correction pattern or a local increase or decrease in pattern size, and the light intensity distribution, so that, in the second conventional defect checking method, it is unavoidable that the minute correction pattern or the local increase or decrease in pattern size is erroneously detected as an imaginary defect.
Secondly, nowadays, to improve the resolution in the vicinity of the wavelength limit of the exposure light of an exposure device such as a stepper, use of a phase shift mask is being considered. The phase shift mask is of various types, including a phase alternate arrangement type (Revenson type), an edge enhancement type, an auxiliary pattern type, a half tone type, a chromeless type, etc. When the second conventional defect checking method is used, the detection of transparent defects of different phases is difficult to perform by using any of these types. Generally speaking, the wavelength of the exposure light is in the wavelength range of from near ultraviolet to ultraviolet rays. The wavelength of the light source used for defect checking is usually in the visible light range. Thus, even in the case of a defect indicating a relatively large phase difference in the range of from the near ultraviolet to the ultraviolet rays, the phase difference decreases with the wavelength with which the defect checking is performed, with the result that it is rather difficult for such a defect to be reflected in the pattern due to the light intensity distribution reproduced with the light used in the defect checking.
Further, in a phase shift mask of the auxiliary pattern type, the auxiliary pattern is generally formed in a size smaller than the wavelength of the exposure light for the purpose of reflecting more faithfully the pattern configuration obtained by deleting the auxiliary pattern on the wafer. Thus, a difference corresponding to the auxiliary pattern is inevitably generated between the light intensity distribution obtained from such a pattern and the pattern data, so that, with the second conventional defect checking method, it is unavoidable that an imaginary defect is erroneously detected as a defect even when there is no defect.
In addition, in a phase shift mask in which a phase shift section is formed by engraving a glass substrate, a reflection and interference of light (a physical phenomenon which is generally called waveguide effect) occurs on the side wall of the phase shift section, with the result that a difference is generated between the pattern data and the pattern reproduced as a light intensity distribution, thereby making the defect checking difficult.
Apart from this, use of a phase shift mask of the half tone type, which is effective for an isolated pattern, is being actively considered. In a half tone type phase shift mask, a pattern is formed which is composed of a translucent region which allows transmission of approximately 4% to 20% of light and a light transmitting region. The light transmittance of the translucent region is set in accordance with the wavelength of the exposure light of the stepper or the like. However, in the wavelength of the visible light used for defect checking, the light transmittance of the translucent region increases. As a result, no light intensity distribution is obtained in the defect checking, so that the defect checking itself is often made impossible.
Thirdly, nowadays, to improve the resolution near the wavelength limit of the exposure light of an exposure device such as a stepper, methods are being considered according to which, the configuration of the exposure light source is varied, or a filter is inserted in the pupil plane of the lens. In these methods, the pattern often requires a correction pattern or an auxiliary pattern, so that the same problem as described above is entailed.
It is accordingly an object of the present invention to provide a pattern defect checking method which is affected neither by the presence of a correction pattern or an auxiliary pattern nor by the waveguide effect and which makes it always possible to reliably detect a defect in a pattern formed on a photo mask regardless of the type of photo mask.